docs: def cd_previous(self):
          """
          cd to the gDirectory before this file was open.
          """
          if self._prev_dir is None or isinstance(self._prev_dir, ROOT.TROOT):
              return False
          if isinstance(self._prev_dir, ROOT.TFile):
              if self._prev_dir.IsOpen() and self._prev_dir.IsWritable():
                  self._prev_dir.cd()
                  return True
              return False
          if not self._prev_dir.IsWritable():
              # avoid warning from ROOT stating file is not writable
              return False
          prev_file = self._prev_dir.GetFile()
          if prev_file and prev_file.IsOpen():
              self._prev_dir.cd()
              return True
          return False